1. Field of the Invention
The present invention relates to thinned aperiodic antenna arrays with improved peak sidelobe level control and, more particularly, to thinned aperiodic antenna arrays having the antenna elements pseudorandomly positioned within the array in a manner prescribed in accordance with an equation based on difference sets to provide improved peak sidelobe level control.
2. Description of the Prior Art
A variety of array antennas are well known in the prior art and include several types. One type is the linear phased array antenna of which the simplest design is the "filled" periodic array which comprises a line of antenna elements with equal excitation power and with an interelement spacing typically of approximately one-half wavelength of the transmitted or received signal. The cost of large linear phased array antennas can be reduced through "thinning" which corresponds to a reduction in the number of elements in the aperture below that of the filled array, and, in turn, an interelement spacing of more than one-half wavelength. However, the more a linear phased array is thinned the less control is generally available to a designer of the radiation pattern in the sidelobe region, which in turn influences the level of the peak sidelobe.
Radiation patterns in slightly thinned linear phased arrays have been improved somewhat by the nonuniform spacing of the reduced number of elements in accordance with certain equations. In this regard see, for instance, U.S. Pat. Nos. 3,130,410 and 3,605,106 issued to F. S. Gutleber on Apr. 21, 1964 and Sept. 14, 1971, respectively, and U.S. Pat. Nos. 3,780,372 and 3,877,033 issued to H. Unz on Dec. 18, 1973 and Apr. 8, 1975, respectively.
U.S. Pat. No. 3,182,330 issued to A. E. Blume on May 4, 1965 improves the radiation pattern by variably spacing the elements of a linear array in accordance with a definite relationship which either progressively spaces the elements further apart or closer together as the distance from a reference point increases. In accordance with the Blume arrangements, echelon lobes usually accompanying components of the radiation in undesired radiation directions are reduced.
U.S. Pat. No. 3,978,482 issued to F. C. Williams et al on Aug. 31, 1976 relates to a dynamically focused thinned linear array which employs variation of a local oscillator frequency together with differential path lengths in the local oscillator lines feeding the various mixer elements at each antenna element along the receiving array to dynamically program the focus of the thinned array. The differential path length spacings between any adjacent pairs of antenna elements varies linearly so that the total path length distribution from the local oscillator to the mixers (assuming a similar distance between each of the mixers and its corresponding antenna element) varies nonlinearly over the entire array.
Some additional sidelobe control may be obtained by applying unequal excitations to the antenna elements. The principle disadvantage of this approach is that the gain of the array will necessarily be less than an array in which full power is applied to all elements.
As described above, thinning can be accomplished by spacing the reduced number of elements either uniformly or nonuniformly in a prescribed manner in the aperture. Another method for thinning a filled linear array using the nonuniform spacing of elements is to randomly locate the reduced number of elements in the aperture. A number of such random designs are then evaluated and the design providing the lowest peak sidelobe is selected. In this regard see the articles "The Peak Sidelobe of the Phased Array Having Randomly Located Elements" by B. D. Steinberg in IEEE Transactions on Antennas and Propagation, Vol. AP-20, No. 2, March 1972 at pp. 129-136 and "An Approach to Peak Sidelobe Control in Moderately Thinned Aperiodic Arrays" by D. G. Leeper in the Valley Forge Research Center Quarterly Progress Report, No. 14, Aug. 15, 1975 at pp. 47-64.
While the equations presented in the above-cited patents produce a number of either small-array designs (10s of elements) or larger nearly-filled arrays with well-controlled peak sidelobe level (PSL) control, the published patterns often showed sidelobes similar in character and level to those of random arrays described in the articles mentioned hereinabove. The similarity has led some designers to conclude that significant sidelobe control may be unobtainable in significantly thinned aperiodic arrays (thinning factor well below one-half).